Destruction of a sign-changing order parameter by artificial atomic defects in multiband superconductor PrOs4Sb12

Chiral superconductivity is a long-sought unconventional state of matter that spontaneously breaks time-reversal symmetry through the development of Cooper pairing with finite angular momentum. Chiral superconductivity is a type of topological state which provides a natural platform for realizing Majorana edge modes being central to various proposals for quantum computation. However, despite intensive theoretical studies and huge experimental efforts, no material has been proven definitively to be a chiral superconductor.

The heavy-fermion and multiband superconductor PrOs$_{4}$Sb$_{12}$, for which a $\mu$SR study [1] and polar Kerr effect measurements [2] showed evidence of broken time-reversal symmetry spontaneously developing below the critical temperature $T_{c}$ $ \simeq$ 1.85 K, is a leading candidate to display chiral superconductivity. Based on measurements of the temperature dependence of the lower critical field $H_{c1}$($T$), we have recently proposed a multiband and multisymmetric scenario, in which a superconducting condensate is composed of a sign-changing smaller gap and a large isotropic $s$-wave gap [3].

To develop a detailed understanding of multicomponent superconductivity in PrOs$_{4}$Sb$_{12}$, we have extended measurements of $H_{c1}$($T$) down to temperatures as low as 7 mK utilizing a 2DEG Hall magnetometry. We observe a sudden increase in $H_{c1}$($T$) deep in a superconducting state, indicative of a rare case of two nearly decoupled bands. Furthermore, a non-saturating and concave behaviour of $H_{c1}$($T$) below about 0.45 K clearly points at a sign-changing symmetry of the smaller gap. Equally remarkable is a high sensitivity of this characteristic to electron irradiation. Indeed, a concentration of artificial atomic defects as small as a few 0.1$\%$ results in both a saturation of $H_{c1}$($T$) at $T$ $<$ 0.15 K and a strong suppression of the anomalous enhancement below $\simeq$ 0.25 $T_{c}$, consistent with a destruction of an unconventional order parameter due to the smaller gap. In addition to this, theoretical description of possible symmetries of the smaller gap as well as results of a comparative study on the two-band isotropic $s$-wave homologue LaRu$_{4}$As$_{12}$ will be discussed in the context of a putative chiral spin-triplet pairing state in PrOs$_{4}$Sb$_{12}$ [4].

[1] Y. Aoki $\textit{et al}$., Time-Reversal Symmetry-Breaking Superconductivity in Heavy-Fermion PrOs$_{4}$Sb$_{12}$ Detected by Muon-Spin Relaxation. Phys. Rev. Lett. $\textbf{91}$, 067003 (2003).
[2] E. M. Levenson-Falk $\textit{et al}$., Polar Kerr Effect from Time-Reversal Symmetry Breaking in the Heavy-Fermion Superconductor PrOs$_{4}$Sb$_{12}$. Phys. Rev. Lett. $\textbf{120}$, 187004 (2018).
[3] J. Juraszek $\textit{et al}$., Symmetry of Order Parameters in Multiband Superconductors LaRu$_{4}$As$_{12}$ and PrOs$_{4}$Sb$_{12}$ Probed by Local Magnetization Measurements. Phys. Rev. Lett. $\textbf{124}$, 027001 (2020).
[4] V. Kozii $\textit{et al}$., Three-dimensional Majorana fermions in chiral superconductors. Sci. Adv. $\textbf{2}$, e1601835 (2016).